No subscriptions and error on subscribeToTags for push notifications (iOS 8)

We're using Bluemix push notifications for iOS 8 with tags. Meaning we're registering the device token when available and displaying available tags and related subscriptions as table with switches. That works most of the time. But after a while without closing the app the available subscriptions suddenly return 0 (zero) although there should be active subscriptions and trying to subscribe again results in errors saying "Expected status code in (200-299), got 400" with message "Invalid value was provided. Check the 'deviceId' parameter value". After restarting the app the available subscriptions are still 0 (zero), but subscribing to tags will work again. Is this a known issue or are we doing something wrong here?

We're using IMFCore 1.0 and IMFPush 1.0 frameworks through CocoaPods. The project is done with Swift 1.2 (Xcode 6.4).

@JanIsLearning
Please provide response to the following questions:
1. what authorization are you using? Google/Facebook/Custom/none ?
2. Have you written any code in their app, that tinkers with or clears the device's keychain?

1: Currently we're not using any authorization at all for push service communication. 2: We do use the keychain for saving, retrieving and deleting single accounts (custom service related) via SSKeychain from Sam Soffes (https://github.com/soffes/sskeychain). Are there any known issues concerning keychain usage?